1. Field of the Invention
The present invention relates to a method and an apparatus for absorbing wave energy and generating electric power by wave force, more particularly to a resonant type method and apparatus for absorbing wave energy and generating electric power by wave force, wherein a pendulum with a pressure-receiving plate is reciprocally swung or rocked by stationary waves in a caisson and the reciprocal movement of the pendulum is converted to electric power.
2. Description of the Prior Art
The inventors had formerly invented a basic apparatus wherein an incident wave is reflected from a back plate of a caisson to yield a reflected wave which is subsequently overlaid by a next incident wave to form a synthesized or multiplicated wave which is called as stationary wave and a pendulum arranged in the caisson at the node of the stationary wave is reciprocally driven or swung by the stationary waves to convert wave energy to useful available energy at high efficiency with a simple structure and cheap investment and maintainance costs and filed therefor a U.S. patent application Ser. No. 238,523, British patent application Ser. No. 8,106,678 and a Canadian patent application Ser. No. 372,213, which is not issued on U.S. Pat. No. 4,400,940, British Pat. No. 2,071,772 and Canadian Pat. No. 1,164,767. The descriptions of the patents are stated herein by references.
The structure of the basic apparatus of U.S. Pat. No. 4,400,940 is shown in FIGS. 1 and 2.
Equation of motion regarding the pendulum 7 in the basic apparatus as shown in FIG. 2 is expressed as follows: ##EQU1## wherein, I: moment of inertia of the pendulum in regard to the swinging center O.sub.P (including those portion caused by additional water mass),
.theta.: swinging angle of the pendulum PA0 .theta.: velocity of swinging angle of the pendulum, PA0 .theta.: angular acceleration of the pendulum, PA0 N.sub.1 .theta.: moment of the pendulum by generating waves, PA0 N.sub.2 .theta.: resistive moment of the hydraulic pressure cylinder or piston and cylinder assembly 10 (caused by orifice or the like restriction to the output of the cylinder), PA0 K.theta.: righting moment of the pendulum (caused by displacement of centroid or the like), PA0 Fw: amplitude of swinging moment caused by wave force, and PA0 Tw: wave period. PA0 (i) making the swinging period T.sub.p of the pendulum to coincide with wave period Tw, thus creating a resonant state, PA0 (ii) making N.sub.1 =N.sub.2. At a given wave force condition, N.sub.1 is represented by a constant. Namely, resistive moment N.sub.2 .theta. caused by the hydraulic cylinder 10 should be proportional (linear resistance) to the swinging velocity .theta. of the pendulum 7, PA0 (iii) arranging the pendulum 7 at the position of the node of the stationary wave.
The inventors have made many researches and experiments to find out that the energy which actuates the hydraulic pressure cylinder 10 can be made maximum (i.e. the rate of absorbing the energy of wave force is maximum) at the following conditions of:
In the apparatus shown in FIG. 2, if T.sub.p is made equal to Tw or T.sub.p =Tw by e.g. adjusting the position of the centroid of the pendulum 7 relative to a given wave force condition and characteristic property of the restriction such as orifice of the output of the hydraulic pressure cylinder is approached to a laminar flow such that the magnitude of the output becomes N.sub.1 =N.sub.2 and fixed thereto, the wave force energy can be converted to thermal energy most efficiently.
The basic apparatus of U.S. Pat. No. 4,400,940 is superior to conventional apparatuses in that it has in water no bearings and the other parts which necessitate maintainance, and it is simple in structure so that investment and maintainance costs are cheap especially when converting wave force energy to oil pressure energy by using the hydraulic pressure cylinder 10.
Afterwards, the inventors had improved and developed the basic apparatus to complete another invention and filed it as a Japanese patent application No. 22,822/81 (Japanese patent application laid-open No. 137,655/82). The invention of the application No. 137,655/82 is superior to the basic apparatus in that it acquires energy of good quality, especially in that it converts wave force energy economically to A.C. current of constant frequency and good quality which can be connected to the public electric power network.
The invention of No. 137,655/82 is a method of generating electric power by wave force characterized in that a hydraulic pressure cylinder is actuated by a pendulum which is swung by waves to discharge oil from the cylinder, a synchronous or induction generator has been preliminarily connected to the public electric power network for revolution at a constant revolution rate, the generator is actuated by at least one oil pressure motor which uses the discharged oil as a pressure source, and a volume to be expelled by the oil pressure motor is varied in proportion to a pressure acting on the hydraulic pressure cylinder, whereby a load acting on the pendulum caused by the cylinder is made proportional to a swinging velocity .theta. of the pendulum and the magnitude of the load acting on the pendulum is made equal to the moment of the pendulum by generating waves.
The invention of No. 137,655/82 relates also to an apparatus for generating electric power by wave force, comprising at least one caisson having a bottom plate, a back plate and side plates opening in an opposite side of the back plate and opening at least a part of an upper portion of the caisson to form therein a water chamber, the caisson being arranged to form at least structural element or part of a breakwater, bank and the like facing to the sea, the water chamber having a length Bc' which is longer than 1/4 of the wave length Lc in the water chamber to generate stationary wave in the water chamber, and a pendulum having a natural period T.sub.p in swinging or rocking substantially the same as periods T.sub.w of stationary wave of water caused in the water chamber and arranged in the caisson at a node position i.e. at a distance from the back plate one fourth or Lc/4 of the length Lc of the wave length in the water chamber so as to be swung by the stationary wave thereby to absorb wave energy to convert it into electric or heat energy, characterized in that the apparatus comprises a hydraulic pressure cylinder actuated by the pendulum to discharge oil therefrom, at least one oil pressure motor connected to the hydraulic pressure cylinder and driven by the oil discharged from the cylinder to expel a volume which is proportional to the discharged oil pressure, and a synchronous or induction generator preliminarily connected to the public electric power network for revolution at a constant revolution rate, a load acting on the pendulum caused by the hydraulic pressure cylinder being proprtional to the swinging velocity .theta. of the pendulum, and the magnitude of the load acting on the pendulum being equal to the moment of the pendulum by generating waves.
However, improvements of the apparatus of No. 137,655/82 have still been earnestly desired and requested to improve its strength such that it has (i) a sufficient strength to withstand extraordinary sea phenomenon such as typhoon, as well as (ii) economicity that can substitute for conventional energies.
The inventors have previously filed a Japanese patent application No. 60,869/82 (patent application laid-open No. 178,879/83) for improving the invention of No. 137,655/82, and is directed to an improvement of the quality of the electric energy to be obtained just similarly as the invention of No. 137,655/82, in that it removes periodical fluctuation of the output of the electric power generated by wave force due to periodicity of the wave force energy, and is not directed to an improvement of the strength of the apparatus such that it can withstand an extraordinary sea phenomenon such as typhoon. In addition, though the invention of the No. 178,879/83 has a second purpose of maintaining the efficiency of the electric generation as high as possible over a wide range of wave height so as to improve the above point (ii), a further improvement of the economicity of the apparatus has still been desired, while improving the stremgth of the apparatus as described in the above point (i).
Wave energy is approximately proprtional to square of wave height. Generally, wave height at the time of typhoon is about 5 times of wave height of usual climate, so that wave energy at the time of typhoon reaches to about 25 times of that of usual climate. From a necessity of withstanding wave pressure at the time of typhoon, the apparatus had to be constructed very strong and tough, so that construction cost became increased and operation efficiency at usual climate became lowered. So, there was a dilemma always.
Therefore, heretofore, an apparatus for generating electric power from wave energy has been desired which is simple in structure and cheap in investment and maintainance costs and yet can withstand an extraordinary sea phenomenon such as typhoon and still has a high operation efficiency at usual climate.